High speed kinetic energy storage flywheel systems provide fast electrical power for load levelling and peak shaving, in competition with battery based systems.
Typically, such an energy storage flywheel system includes a high speed rotating flywheel (usually made of high strength steel or carbon), an integral motor/generator unit and a power conversion unit (to convert mechanical power to electrical power or conversely to convert electrical power to mechanical power). The stored energy per flywheel is typically up to 10 kWh. The peak power provided by the motor/generator power conversion system can vary between 10 kW and several hundred kW.
For such a flywheel based storage system it is important to maximize operating efficiency:
All rotating components are contained within a vacuum enclosure in order to minimize the windage losses.
The rotating components are advantageously operated contactless on active magnetic bearings. Practically, on a vertically arranged flywheel the total losses of a properly adapted 5-axis magnetic bearing system are less than 100 W.
Applied to a highspeed flywheel, magnetic bearings imply no maintenance, minimum rotor- and bearing losses, no wear, no lubrication and have substantially unlimited lifetime.
However a drawback of such a maximized operating efficiency is a very long unassisted spin down time.
A magnetically levitated flywheel system includes a backup bearing system to support the high speed rotating parts in case of a fault or a failure in the magnetic bearing system. The duration of a spin down in the backup bearings can vary as the generator through the power conversion unit can also be used for faster deceleration. However, as a worst case scenario, an unassisted spin down (i.e. without braking from the generator) in the backup bearings could occur. The unassisted spin down time in the backup bearings is many hours.